Aqueous fluoropolymer dispersions have been widely used for various applications such as linings of cooking utensils and pipes, or impregnated glass cloths because the dispersions can be formed into films showing excellent chemical stability, non-tackiness, weather resistance and the like, by a method of coating, impregnation, or the like. In such applications, the aqueous fluoropolymer dispersion preferably has a high fluoropolymer concentration. From this standpoint, commonly used dispersions are those obtained by polymerizing fluorine-containing monomers in an aqueous medium in the presence of a fluorine-containing anionic surfactant, and concentrating the resulting polymers. In terms of the cost, however, the fluorine-containing anionic surfactant is preferably removed from the aqueous fluoropolymer dispersion.
Aqueous fluoropolymer dispersions containing only a small amount of a fluorine-containing anionic surfactant, however, have following problems: (1) an increase in the fluoropolymer concentration increases the viscosity of the dispersion, leading to poor handleability; and (2) fluoropolymer particles tend to be agglomerated when subjected to mechanical stress during transport or processing such as film formation, lowering the quality of the aqueous dispersions and of films or impregnates formed therefrom (i.e., problem of mechanical stability). To solve the problems, aqueous fluoropolymer dispersions excellent in dispersion stability even with a low concentration of a fluorine-containing anionic surfactant have been developed (see, for example, Patent Literatures 1 to 3).
Patent Literature 4 discloses addition of an inorganic metal salt, such as potassium sulfate, to an aqueous fluoropolymer dispersion for the purpose of avoiding gelation of the fluoropolymer in the aqueous fluoropolymer dispersion that is standing still.
Patent Literature 5 discloses use of a mixture of a nonionic non-fluorinated surfactant and a non-fluorinated anionic surfactant for the purpose of solving a problem that the stability of a concentrated dispersion system may be lower, under specific conditions, than that of a dispersion system containing a larger amount of a low-molecular fluorinated surfactant. A preferable non-fluorinated anionic surfactant disclosed in Patent Literature 5 is an anionic hydrocarbon surfactant having a sulfonate group.
A aqueous fluoropolymer dispersion obtained by polymerization of fluoromonomers contains a polymerization initiator decomposed during the polymerization reaction, and therefore is commonly acidic with a pH value of about 1.0 to 3.0. Accordingly, the aqueous fluoropolymer dispersion obtained by polymerization is concentrated by a known method to have a higher fluoropolymer concentration and adjusted to be alkaline using an alkali such as ammonium or hydroxide ions, before its use.